OEChem‘s aromaticity perception routines are based around the Hückel‘s
rule that defines cyclic conjugated systems with \((4N+2)\) number of
\(\pi\) electrons as aromatic (where \(N\) is zero or any positive integer).

However, when a molecule is imported from a file with a high-level
OEReadMolecule function,
atom and bond aromaticity is automatically perceived using the default
OEAroModelOpenEye model.

As mentioned before (in section Molecular Property Preservation), the
high-level OEWriteMolecule` writer function
may automatically update atom and bond properties (including aromaticity)
in order to standardize the exported molecules.
The following table shows the aromaticity models associated with various file
formats.